1. Field of the Invention
The invention relates to a method of manufacturing a semiconductor device and, more specifically, to a method of manufacturing a flash memory device.
2. Discussion of Related Art
In a general process of forming a device isolation film of a semiconductor device, a photoresist pattern for forming a device isolation film is formed on a predetermined region of a semiconductor substrate and a trench is formed by performing an etching process of micro-etching the photoresist pattern. At this time, an oxidation process of forming a side wall oxide film on the side wall of a trench is performed to compensate for etch damage generated from the etching process, make upper or lower edge of the trench rounded, and increase adhesive force of an insulating film to be buried in the inside of the trench. The oxidation process is performed at the temperature of about 1000° C.
At this time, when ion implantation for adjusting a threshold voltage is performed on the semiconductor substrate through an ion implantation process before a process of forming a device isolation film, the oxidation process makes the implanted ions for adjusting a threshold voltage diffuse into the side wall oxide film.
Accordingly, an active region has non-uniform distribution of ion density due to the diffusion of the implanted ions for adjusting a threshold voltage into the side wall oxide film and the non-uniform distribution of ion density deteriorates the performance of the device by generating humps, increasing a leakage current, and generating an inverse narrow width effect which lowers the threshold voltage.